The present invention relates, in general, to metallization systems and, more particularly, to copper metallization systems.
Monolithically integrated circuits typically include multiple layers of metal that terminate in bondpads through which electrical signals are transmitted. In the past, the multiple layers of metal have been formed from aluminum. However, semiconductor manufacturers have begun using copper, rather than aluminum, because of copper""s superior electromigration performance as well as its lower resistivity. In addition, a copper metallization process can actually lead to lower manufacturing costs than aluminum.
Although there are many advantages to using copper, its surface is not suitable as a terminal metal for packaging interconnections. Separate coating metals are needed to deposit onto the copper bond pads for packaging interconnections. One method is direct deposition of metal onto copper bond pads by an electroless deposition technique such as electroless nickel followed by immersion in gold for flip chip applications and/or electroless palladium deposition followed by immersion in gold for wirebond applications. For such deposition techniques, the copper surface is generally activated with a very thin layer of palladium in a palladium activation bath to allow deposition of a nickel layer thereon. In this step, the palladium covers the copper so that nickel can be electrolessly plated on the layer of palladium. When the palladium only partially covers the copper, a galvanic cell is set up that results in preferential etching at the copper-palladium grain boundaries. The preferential etching causes voids in the copper and undercutting at the interface between the copper and palladium.
FIG. 1 is a highly enlarged cross-sectional view of a semiconductor wafer 10 having a copper bondpad 11 disposed thereon. A layer 12 of nickel is disposed on copper bondpad 11. Copper bondpad 11 has voids 13 at the interface between the copper and nickel layers caused by the use of palladium to activate the copper surface. Voids 13 lower the shear values of the interface, increase the probability of sodium contamination, and create electromigration problems.
Accordingly, it would be advantageous to have a method for electrolessly plating metals on a copper surface that is cost efficient and suitable for use in an integrated circuit manufacturing process. It would be desirable to have a semiconductor structure on which a copper bondpad is disposed that is void-free, i.e., does not have voids, and a method for manufacturing this structure. It would be of further advantage for the method to be suitable for use at temperatures less than about 90 degrees Celsius.